After light is polarized, it moves only in one direction. This is called linear polarization.
Understanding Polarized Light
In a classic optical microscope, also known as the simple "light" microscope, visible light is sent through a series of lenses to magnify a sample. A polarizing microscope is an optical microscope with the addition of a polarizing device that acts to polarize visible light. To understand how a device polarizes light, it is helpful to first know what polarized light is.
According to the wave model of light, visible light is composed of two waves vibrating at right angles to each other along the direction the light is traveling. In other words, regular light vibrates in all directions. When light is polarized, it is forced to vibrate in one direction. Technically, polarizers convert circularly polarized light to linearly polarized light.
Understanding Polarizers
Light can be polarized with the use of polarizing filters, also known as polarizers. Polarizers only let light vibrating along their polarizing axis pass through completely. These filters have particles that are aligned in one direction entirely, and absorb light waves moving in other directions. When a polarizing filter is exactly 90 degrees from its set polarizing axis, it blocks all light from passing.
A polarizing microscope uses two polarizing filters, a polarizer and an analyzer. The polarizer is located under the stage (the small platform where samples are placed), and is usually fixed to a left-right, or East-West position. The analyzer is mounted above the stage, and is set to a North-South position, though it can be rotated on some microscope models. The analyzer can be slid in and out of the path of the source light (which is located at the base of a microscope).
When the source light is turned on, light moves upwards and is polarized to move in one direction by the polarizer. It is now traveling up, shining through our sample, its light vibrating back and forth in an East-West position. It will continue upwards undisturbed if the analyzer is not in use. When the analyzer is pushed in, only light moving in a North-South position is allowed to move through. Because all of our light was already polarized to move in an East-West position only, no light can pass. Our polarizers are now "crossed," and the viewer will only see darkness.
Uses of Polarizing Microscopes
Microscopes with polarized configurations are commonly used in petrography to study the optical properties of rocks and minerals. Using the polarizer by itself will assist in studying the sample under polarized light, while using both filters (polarizer and analyzer) allow study under cross-polarized light. While no light can pass under cross-polarized light, it is possible to discern mineral characteristics. Minerals within a sample are generally aligned at various angles, so as one rotates the stage, different parts will "black out" at different times.
Tags: polarized light, East-West position, only light, visible light, also known, cross-polarized light